Television

Television, communication of moving pictures between distant points over wire or by means of electromagnetic waves. In television broadcasting, centrally prepared programs are transmitted to millions of individual receivers. Closed-circuit transmissions, which rely upon signals carried over wire rather than electromagnetic waves broadcast at large, are most often used for industrial and educational applications.

The moving picture on a television screen originates with a television camera, which forms an optical image of the scene to be transmitted and then breaks the image down into electrical signals. The signals may be amplified and transmitted directly over a cable, or they can be converted into electromagnetic waves. As electromagnetic waves, they are transmitted by antennas, like radio waves, picked up by receiving antennas, and then conveyed to a television set equipped to reconstitute the electromagnetic waves into an optical image on the screen of a cathode-ray tube.

In the early days of television technology, most cameras used the iconoscope, but the iconoscope technology has since been replaced by orthicon or vidiscope. In order for an image to be transmitted and received electronically, it must be broken down into discrete but organized electrical signals and then reassembled or reconstructed by the receiver. The image formed by the optical lens system of a television camera is scanned as a sequence of 525 horizonal lines. As an image is scanned, variations in the light intensity along each line are converted into a fluctuating electrical signal, the brightness signal, which gives the image its gradations from light to darkness. Each scan is then repeated 30 times a second to create the illusion that the image is moving and without noticeable flicker. When reassembled on the television screen, the image appears whole and in fluid motion because of the persistence-of-vision effect of the human eye.

In color television, the image is not only resolved into brightness, but into hue and saturation as well. The light entering the camera is analyzed into red, green, and blue components, the 3 primary colors. Both hue and saturation are converted to electronic information, which is added to the brightness signal and transmitted. In the color receiver, this information is recovered and used to control 3 electron beams fired within the television set. The beams are projected through a shadow mask, a screen containing some 200,000 minute, precisely positioned holes, and they excite the mosaic of red, green, and blue phosphor dots, which reproduce the color image on the television screen.

The possibility of television was conceived in the early days of the electric telegraph, but the realization had to await several key developments. First was the discovery of the photoconductive properties of selenium, followed by the development of the cathode-ray tube in 1897 and the electron tube in 1904. Crucial to both systems and at the heart of television technology was scanning. The first practical television system, demonstrated in London in 1926 by J. L. Baird, used a mechanical scanning method devised by Paul Nipkow in 1884. Electronic scanning dates from 1923 when Vladimir Zworykin filed a patent for his iconoscope camera tube. Television broadcasting began in London in 1936 using a 405-line standard. In the United States, public broadcasting began in 1941, and regular color broadcasting in 1954. U.S. television broadcasts are made in the VHF (Channels 2–13) and UHF (Channels 14–83) regions of the RF spectrum. Recent technology, both in Japan and the United States, has led to the development of High Definition TV with greatly increased scanning capacities making possible images that are much cleaner and much larger.

See also: Sarnoff, David; Videodisc; Videotape recorder.